This invention relates to cardiac stimulating devices and particularly to implantable cardiac stimulating devices capable of providing rate-responsive pacing therapy. More particularly, this invention is directed toward an accelerometer-based, multi-axis physical activity sensor for measuring levels to which a patient is engaged in physical activity, so that rate-responsive pacing therapy can be administered accordingly.
A pacemaker is an implantable medical device which delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia--a condition in which a patient cannot maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate, which was typically set at a rate deemed to be appropriate for the patient at rest. The predetermined rate was usually set at the time the pacemaker was implanted, and in more advanced devices, could be set remotely after implantation.
Early advances in pacemaker technology included the ability to sense a patient's intrinsic cardiac activity (i.e., the intercardiac electrogram, or "IEGM"). This led to the development of "demand pacemakers," so named because these devices deliver stimulation pulses only as needed by the heart. Demand pacemakers are capable of detecting a spontaneous, hemodynamically effective, cardiac contraction which occurs within a predetermined time period (commonly referred to as the "escape interval") following a preceding contraction. When a naturally occurring contraction is detected within the escape interval, a demand pacemaker does not deliver a pacing pulse. The ability of demand pacemakers to avoid delivery of unnecessary stimulation pulses is desirable, because it extends battery life.
Pacemakers such as those described above proved to be extremely beneficial in that they successfully reduced or eliminated seriously debilitating and potentially lethal effects of bradycardia in many patients. However, the early devices were not adjustable "in the field"--that is, the heart rates maintained by these devices were not adjustable in accordance with changing levels of physical exertion. Thus, during periods of elevated physical activity, some patients were subject to adverse physiological consequences, including light-headedness and episodes of fainting, because their heart rates were forced by the pacemaker to remain constant at an inappropriately low rate. Also, some patients were subject to discomfort resulting from heart rates that were maintained higher than would normally be appropriate during periods of rest.
A major advance in pacemaker technology was the development of "rate-responsive pacemakers." These devices are capable of adjusting the patient's heart rate in accordance with metabolic demands, even as those demands vary as a result of changing levels of physical exertion. Rate-responsive pacemakers typically maintain a predetermined minimum heart rate when the patient is engaged in physical activity at or below a threshold level, and gradually increase the maintained heart rate in accordance with increased levels of physical activity until a maximum rate is reached. In many rate-responsive pacemakers, the minimum heart rate, maximum heart rate, and the slope or curve between the minimum heart rate and the maximum heart rate are programmable, so that they may be configured to meet the needs of a particular patient.
In order to provide rate-responsive pacing therapy, a pacemaker must be capable of correlating an indicator of physical activity to an appropriate heart rate. Past efforts to identify a reliable indicator of physical activity have led to the investigation of several physiological parameters--many of which have proven to be unsatisfactory in the context of rate-responsive pacing. Some of the physiological parameters that have been studied include central venous blood temperature, blood pH level, QT time interval, respiration rate, thoracic impedance, central venous oxygen saturation, stroke volume, and nerve activity. However, these indicators exhibit certain drawbacks with respect to their use in connection with rate-responsive pacing, including slow response time, excessive emotionally induced variations, and wide variability across individuals. Accordingly, these physiological indicators have not been widely applied in practice.
More generally accepted have been rate-responsive pacemakers which employ sensors that transduce mechanical forces associated with physical activity. A widely used sensor of this type incorporates a piezoelectric crystal which generates a measurable electrical potential when a mechanical stress resulting from physical activity is applied to the sensor. Dahl U.S. Pat. No. 4,140,132 and Anderson et al. U.S. Pat. No. 4,428,378 describe examples of rate-responsive pacemakers that maintain a patient's heart rate in accordance with physical activity as measured by a piezoelectric sensor.
Despite the widespread use of piezoelectric sensors in rate-responsive pacemakers, certain difficulties associated with their use have become apparent. For example, sensors that employ piezoelectric crystals typically provide extremely small output signals which are difficult to process. Also, assembly of a sensor incorporating a piezoelectric crystal is difficult because handling can cause stresses which exceed the tolerance limits of the crystal. The process of securing the sensor to a suitable supporting structure in the pacemaker can cause unacceptably high stresses, which can lead to fracturing of the crystal.
These and other difficulties related to the use of a piezoelectric sensor in a rate-responsive pacemaker were addressed in commonly assigned, copending U.S. patent application Ser. No. 08/059,698, filed May 10, 1993, entitled "A RATE-RESPONSIVE IMPLANTABLE STIMULATION DEVICE HAVING A MINIATURE HYBRID-MOUNTABLE ACCELEROMETER-BASED PHYSICAL ACTIVITY SENSOR FOR A RATE-RESPONSIVE PACEMAKER AND METHOD OF FABRICATION." That application describes a novel physical activity sensor that employs a resilient piezoelectric polymer in the transducing element. A cantilever beam of the sensor, which incorporates the piezoelectric polymer, is able to deflect to a greater extent than would be the case if a piezoelectric crystal was used and accordingly, the sensor provides a stronger output signal. Also, the resiliency of the piezoelectric polymer reduces the likelihood of fracturing during the fabrication process.
Despite the advances made as described in the commonly assigned, copending U.S. patent application mentioned above, certain additional difficulties have remained unaddressed. One of the most pressing of these difficulties is related to directional sensitivity--that is, physical activity sensors which have been widely used in the rate-responsive pacing context have been designed so as to be responsive to physical activity in directions along a single axis. U.S. Pat. No. 4,140,132 (Dahl), mentioned above, illustrates a sensor having such limited directional sensitivity--the axis of sensitivity being the one that projects from the patient's chest.
One of the main difficulties associated with the use of single-axis sensors is that they impose limitations on a physician's ability to choose the most appropriate axis of sensitivity for a particular patient. The common design choice has been the axis that projects from the patient's chest (as described above with respect to U.S. Pat. No. 4,140,132 (Dahl)). However, a physician may decide that another axis of sensitivity is more appropriate for a particular patient, for instance, in a case where a patient is frequently subjected, perhaps for occupational reasons, to externally induced forces in directions along the more commonly selected axis of sensitivity.
One might think that a typical prior art single-axis physical activity sensor could provide the desired flexibility if it were simply oriented in different directions as prescribed by a physician. Unfortunately, the solution is not so simple, mainly because of the frequently encountered problem of "twirler's syndrome"--a condition where the patient absent-mindedly manipulates the pacemaker implanted beneath the skin, thereby changing its orientation from time to time. If, for example, a single-axis sensor is initially positioned so as to be sensitive to physical activity in directions along the vertical axis (i.e., the axis extending along the length of the patient's body when the patient is standing), any twirling of the pacemaker will lead to a corresponding change to the axis of sensitivity, such that the new axis of sensitivity is likely to be one other than the vertical axis. Changes to the axis of sensitivity may lead to unexpectedly high or low measurements of physical activity which can cause the pacemaker to make inappropriate heart rate adjustments.
One concern that must always be kept in mind when designing pacemaker components is the need to conserve limited space available within the pacemaker. There is tremendous demand for implantable cardiac stimulating devices, including pacemakers, of reduced size but increased functionality. Thus, both the size and the number of components required to construct a physical activity sensor should be kept to minimum, and accordingly, any attempt to improve directional sensitivity should avoid the use of additional hardware components, including additional transducers or wires, to the greatest extent possible.
What is needed therefore is a physical activity sensor that is suitable for use with a rate-responsive pacemaker and which provides improved directional sensitivity. The improved sensor should overcome deficiencies associated with single-axis physical activity sensors, while minimizing the number of additional components required. In addition, the improved sensor should provide a relatively strong output signal and should be manufacturable in an efficient and cost-effective manner. Further, the sensor should be easy to secure to a suitable substrate--in particular, the pacemaker hybrid, so that the assembly and installation of the sensor can be conveniently integrated to the hybrid manufacturing process. The sensor should also be resistant to breakage, both during fabrication and in use.